1. Technical Field
This invention is related to photonic devices, and more particularly, to optical fiber-based time delay devices.
2. Related Technology
Optical-domain, radio-frequency (RF) true-time-delay (TTD) lines with programmable time delays, wide bandwidth and low optical loss are important components of microwave photonic signal processing systems and future optical communications networks. Their unique advantages, including low loss (independent of RF frequency), large instantaneous bandwidth, immunity to electromagnetic interference, and parallel signal processing capability, have led to the realization of high-performance, tunable microwave filters, phased array beamformers, fast analog-to-digital converters, arbitrary waveform generators, signal correlators, and frequency converters and mixers. For such applications, it is desired that the delay lines exhibit low loss, wide RF bandwidth, and minimal frequency-dependent loss and dispersion. Furthermore, the ability to continuously-tune a true time delay over many RF periods is of interest for enabling high-resolution, reconfigurable optical-domain signal processing and beam-forming systems.
True-time-delay lines have been implemented using mechanical devices for stretching the optical fibers. In other examples, fiber-optic delay line filters using chirped fiber Bragg gratings have been used. FIG. 1 shows a free space optical system for producing a time delay. An optical signal is sent through a circulator 2, exits the circulator, travels along an optical path that includes a lens 4 and a translatable mirror 6, and returns through the lens 4 and back to the optical circulator 2. Motion of the mirror 6 in a direction along the optical path can extend or shorten the signal's optical path, so that the signal experiences a time delay that can be adjusted by changing the mirror's position.
U.S. Pat. No. 7,003,204 to Upton, U.S. Pat. No. 7,521,395, U.S. Patent Publication No. 2006/0140563 to Upton, and U.S. Patent Publication No. 2006/0140564 to Upton disclose continuously variable optical delay lines with a holey fiber configured in a spiral layout, with one end of the fiber operative to a reflective fluid reservoir, and with a segmented piezoelectric actuator overlaying the spiral coiled fiber employed to position the reflective fluid within the fiber.
Optical-resonance techniques that rely on the concept of slow light also have been proposed to produce time delay effects for optical signals.